Apparatus for welding a cover to a tubular ceramic housing

ABSTRACT

A hermetically sealed enclosure for a semiconductor device and a method and apparatus for making same is disclosed. A tubular ceramic housing is provided with an integral die on one end. A cold weldable layer is brazed on the integral die. A first cover member is brazed onto the other end providing a hermetic seal there. A second cover member is cold welded to the layer hermetically sealing the one end. The cold welding apparatus includes a pressure equalizing assembly having a pair of spaced resiliently coupled elements with facing hemispherical recesses. One of the elements slides on a bearing ball nested within the recesses to equalize the compressive forces of the cold welding around the end of the housing.

United States Patent [1 1 Furnival APPARATUS FOR WELDING A COVER TO ATUBULAR CERAMIC HOUSING [75] Inventor: Thomas J. Furnlval, Logansport,

Ind.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

221 Filed: Mar. 27, 1972 211 Appl.No.: 238,354

Related US. Application Data [62] Division of Ser. No. 60,261, Aug. 3,1970, Pat. No.

[ Aug. 14, 1973 1.??49'1. esem +R sdB lawns Attorney-William S.Pettigrewand R. J. Wallace ABSTRACT A hermetically sealed enclosure fora semiconductor device and a method and apparatus for making same isdisclosed. A tubular ceramic housing is provided with an integral die onone end. A cold weldable layer is brazed on the integral die. A firstcover member is brazed onto the other end providing a hermetic sealthere. A second cover member is cold welded to the layer hermeticallysealing the one end. The cold welding apparatus includes a pressureequalizing assembly having a pair of spaced resiliently coupled elementswith facing hemispherical recesses. One of the elements slides on abearing ball nested within the recesses to equalize the compressiveforces of the cold welding around the end of the housing.

2 Claims, 4 Drawing Figures Patented Aug. 14, 1973 APPARATUS FOR WELDINGA COVER TO A TUBULAR CERAMIC HOUSING CROSS REFERENCE TO RELATEDAPPLICATIONS BACKGROUND OF THE INVENTION This invention relates to anenclosure for a semiconductor device, and to a method and apparatus forfabri- 2O cating such an enclosure.

In order to obtain a hermetic seal at the interface of two ductilemetals by cold welding, a compressive force of about tons or more isoften necessary. Additionally, a close tolerance parallelism between dieand anvil surfaces is usually recommended. Otherwise, the compressiveforce can be applied unequally over the interface and high and lowpressure regions can result. Too high a pressure in one place can causea weak or overstressed cold weld, while too low a pressure often resultsin a pervious or incomplete weld.

Toinsure that a continuous cold weld is obtained, it has often beennecessary to use larger compressive forces with thicker cold weldablepieces. The use of thicker pieces guards against puncture; the use of alarger force insures that the thickness reduction percentage deemednecessary to obtain reliable hermetic seals is accomplished. Thisprocedure, however, can add unwanted expense on the one hand, and canincrease the likelihood of damaging an underlying workpiece on theother. Further, while close tolerance parallelism is desirable it can bedifficult to achieve and expensive to maintain. For example, it may benecessary to refinish the die and anvil surfaces frequently in certaincold welding applications to eliminate surface irregularities which canbe expensive.

An embodiment disclosed in the previously mentioned concurrently filedapplication shows a ceramic housing in which one end is utilized as anintegral die. In. conjunction therewith, a continuous ductile layer hasits innermost portion brazed to the housing, with its outermost portionoverlying the integral die. A rim of a cover member is cold welded tothis layer by a movable anvil cooperating with the integral die toproduce the required compressive forces. This embodiment as disclosedproduces good results.

However, production line yields can be improved if theintegral diesurface is quite flat and parallelism is maintained within a toleranceof less than about 5 mils. Such parallelism allows one to use thinnercold weldable metal pieces and less compressive force. While a ceramicsurface can be provided with this degree of smoothness and end-to-endparallelism by conventional finishing techniques, such a requirement canincrease the overall cost of the enclosure. Besides, subsequenthandling, unless a high degree of care is maintained, can damage .thisfinish.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod of cold welding a cover member to a tubular ceramic housing inwhich close tolerance parallelism between die and opposite end surfacesis not generally required, yet production line yield can be increased.

It is another object of this invention to provide cold welding apparatuswhich substantially uniformly distributes the compressive force over thearea to be cold welded even when the opposite ends of the tubularelement are not closely parallel.

It is still another object of this invention to provide an improvedhermetically sealed enclosure for a semiconductor device.

This invention includes providing a tubular ceramic housing with anintegral cold welding die on one end, brazing a cold weldable layer ontothe integral die, and cold welding the rim of a cover member to the coldweldable layer on the die providing a hermetic seal thereat. The coldwelding apparatus includes a pressure equalizing assembly having a pairof plate elements having facing hemispherical recesses, a bearing ballnested therein, and a coupling assembly for resiliently coupling theplate elements wherein one plate element can slide on the bearing ballthereby equalizing the cold welding compressive forces on the housing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a sectional view of anenclosure and cold welding apparatus used to carry out the invention;

FIG. 2 depicts the cold welding apparatus in pressing engagement withthe enclosure;

FIG. 3 shows an enlarged detailed view of a portion of FIG. 1; and

FIG. 4 shows an enlarged detailed view of a portion of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Attention is directedto the drawings and in particular to FIG. I. It shows a support bed 10,a pressure equalizing assembly 12, front and back welding ringsdesignated 14 and 16, respectively, on opposite ends of an enclosure anda movable anvil 18.

The enclosure includes a tubular ceramic housing 20 with front and backmetallic cover members, there being a semi-conductor device 22 enclosedtherein. Tubular housing 20 which is of aluminum oxide, or the like, hasfront and back annular faces, the inner edge of both being beveled toinhibit spalling. A plurality of spaced apart convolutions extendradially outward from the outer surface of the housing providing ahigher voltage capacity therefor. An integral annular circumferentialprojection 2d extends axially, or perpendicularly, from the front facemidway between its outer and inner edges and terminates in a flat land26. Projection 24 provides a closed integral cold welding die for thehousing.

As is best seen in FIGS. 3 and 4, a copper layer 28 in the form of aclosed ring overlies the entire land surface, it being brazed thereto.Brazing, as herein used, refers to a method of securing two contactingsurfaces together by fusing a metal therebetween at an elevatedtemperature. Continuing, the back surface of the ring is coextensivewith the land.

Turning to the back cover member, it is made of copper and includes athick generally cylindrical contact 30 and a stepped rim 32 which isbrazed around the longitudinal side of contact 30. The outermost part ofrim 32 is brazed to the back annular face of the housing providing ahermetic seal thereat. Contact 30, which has a diameter notsubstantially smaller than the inside housing diameter, includes a backsurface adjacent the back face of the housing and a front surfacelocated centrally therein.

The semiconductor device enclosed within the housing is a disc-shapedrectifier which includes anode and cathode terminals in the form ofcylindrical slugs, labelled 34. and 36, respectively. Terminal 36 restson the front surface of contact 30 and is coextensive therewith.

Referring now to the front cover member, it is also made of copper andincludes a thick generally cylindrical contact 38 and a thin steppedcircumferential radially extending rim 40 brazed around the longitudinalside of the contact. Contact 38, being similar to contact 30, includes aback surface which engages the terminal 34 centrally within the housingand a front surface adjacent the integral die. Rim 40 includes anoutermost section 42 the center portion of which completely overlies thefront surface of layer 28 forming a cold weldable interface 43therebetween.

Discussing now the pressure equalizing assembly 12, it includes a pairof spaced apart steel plate elements 44 and 46, the opposing and facingsurfaces of which are flat. The facing surfaces of each plate havefacing hemispherical recesses labelled 48 and 50, respectively, whichare aligned with each other. Plate 44 has a pair of spaced apartthreaded openings 52, extending part-way therethrough from its facingsurface. Plate 46 has a pair of openings therethrough, each of whichbeing registered with a corresponding threaded opening 52. Each openingthrough plate 46 includes a large diameter section 54 adjacent itsopposing surface and a smaller diameter section 55 adjacent its facingsurface with a shoulder 56 therebetween.

A hard steel bearing ball 58, chrome steel alloy or the like, is nestedwithin the space defined by the facing recesses, the radius of curvatureof the ball being equal to that of the recesses. The plates are slidablyheld against opposite hemispherical portions of the ball by couplingmeans in the form of a bolt 60 within each pair of aligned openings anda spiral-like spring 62 disposed around each bolt. The springs are eachattached at their opposite ends to the facing surface of each plate. Thehead of each bolt, which is of larger diameter than section 55, iswithin section 54 of the openings through plate 46. The opposite end ofeach bolt is in threaded engagement with tapped opening 52 terminatingmidway therein.

Under a no-load or static condition, the opposing and facing surfaces ofeach plate are horizontal. The head of each bolt is containednonengagingly within section 54 adjacent shoulder 56 spaced therefrom apredetermined amount. It should be noted that this predetermined spacingdetermines the amount that the front plate member can slide or pivot onthe bearing ball during load or dynamic conditions.

Next discussing the welding rings, they are also made of a hardenedsteel which can be a chrome alloy or the like. The welding rings, socalled herein because they underlie and overlie the faces of the ceramichousing,

are of annular configuration. The radial width of each should preferablybe at least equal to the radial width of the faces of the ceramichousing. However, acceptable results may be obtained if the width of thefront ring is at least equal to that of the land of the integral die.

Tubular housing 20 has pertinent dimensions which include an insidediameter of 1.36 inches, while the end faces are spaced apart by 630mils. The width of each end face is 120 mils and an integral die extendsfrom the front face 62 mils and is 50 mils in width. The cold weldablemembers, layer 28 and rim 40 are both 25 mils thick which is preferredfor this embodiment. However, as previously mentioned, an importantaspect of this invention resides in the fact that thinner cold weldablemembers can be used. In fact, hermetic seals can be reliably obtainedunder production line conditions with cold weldable member thicknessesof about 14 mils.

Continuing with other dimensions, each plate member is of a disc-shapedconfiguration with a diameter of 4 inches and a thickness of 1.25inches. Each hemispherical groove is centrally located having a depth of500 mils and a 1 inch radius of curvature; while the bearing ball has a2 inch diameter.

A method of hermetically sealing the tubular housing can now bedescribed. With particular reference to FIG. 1, the back welding ring iscentrally located on the front plate. The housing is located coaxiallyon the back welding ring. The front welding ring is also locatedcoaxially with the housing on its front end.

When the front welding ring engages the rim of the front cover member,as shown in FIG. 2, plate 44 slides or pivots about the arcuate surfaceof the ball bearing and assumes an orientation which evenly distributesthe force over the end faces of the housing. Accordingly, any surfaceirregularities and/or lack of parallelism between the compressivesurfaces is compensated for. Moreover, uneven metal flow during the coldwelding operation can also be compensated for by movement of plate 44.When the compressive forces are removed, the spiral-like springsdisposed around each bolt can restore the assembly to its originalposition.

As is generally known, an acceptable cold weld can be obtained if thecombined thickness of the cold weldable members is reduced to about 50percent of their original thickness, which requires a predeterminedcompressive force. Heretofore, if one were using thin cold weldablemembers, a percent reduction of about 80 percent would often be used toinsure a continuous cold weld with the attendant risk of puncture. Theinvention as herein disclosed permits one, in an appropriateapplication, to obtain a reliable hermetic seal with only about a 50percent thickness reduction. This greatly decreases the likelihood ofdamage to an underlying housing. Moreover, since lesser compressiveforce is required to insure a hermetic seal, cold weldable members of athickness of about 40 mils each have been successfully cold welded underproduction line conditions.

It should be noted that although the herein described embodiment hasincluded specific dimensions and has been described with reference to aspecific semiconductor device, no such limitation is intended. Forexample, any suitable semiconductor device, including integratedcircuits can be so enclosed. Further, other cold weldable materials suchas aluminum and alloys of copper and aluminum can be used for the covermembers and the ductile layer. However, copper andparticularlycommercial oxygen-free high conductivity copper ispreferred.

It should also be noted that although the integral cold welding die ofthis invention has been described as an annular projection such alimitation is not intended. For example, any closed, or continuous,circumferential configuration can be acceptable. In fact, one entire endof the housing could constitute the integral die. Moreover, although thecold weldable layer as herein described preferably overliescoextensively the land of the integral die, such need not be the case.If necessary, the layer need only overlie a portion of the land.However, if one uses a layer width of less than about onehalf the radialwidth of the land in this embodiment, the likelihood of reliablyproviding hermetic seals under production line conditions can bedecreased.

Although this invention has been described in connection with certainspecific examples thereof, no limitation is thereby intended except asdefined in the appended claims.

I claim: I

1. In a cold welding apparatus for circumferentially cold welding acircular cover member against one end face of a tubular ceramic housinghaving opposite end faces in which said one end face of said ceramichousing has an integral annular cold welding die formed thereon, ringmeans for registration with said integral annular cold welding die onsaid ceramic housing and for selectively applying cold welding pressureto said die throughout its circumference, a pressure equalizing assemblywhich comprises a pair of spaced plates having generally aligned facinghemispherical recesses, a bearing ball nested within said recesses, theradius of curvature of said bearing ball being generally equal to thatof the recesses, ring means on one of said plates for supporting theopposite end face of the housing, and means for resiliently retainingsaid plates together yet allow said one plate to pivot on said bearingball in order to equally distribute a compressive force on the end facesof the tubular ceramic housing, while the other plate remains stationaryon a support.

2. In a cold welding apparatus for circumferentially cold welding acircular cover member against one end face of a tubular ceramic housinghaving opposite end faces in which said one end face has an integralannular cold welding die formed thereon, ring means for registrationwith said integral annular cold welding die on said ceramic housing andfor selectively applying cold welding pressure to said die throughoutits circumference, a pressure equalizing assembly which comprises spacedupper and lower generally circular plates having mutually facing andopposite surfaces, said facing surfaces of said plates having generallyaligned hemispherical recesses, said upper plate having threaded boresextending perpendicularly into said facing surface and only partiallythrough said plate, the lower plate having passageways therethroughperpendicular to its facing surface and in register with said threadedbores, each of said passageways having a smaller diameter portionintersecting the facing surface of said lower plate and a largerdiameter portion intersecting the opposite surface of saidlower plate, abearing ball nested within said recesses on said facing surfaces of saidplates, the radius of curvature of said bearing ball being generallyequal to that of the recesses, ring means on said opposite face of saidupper plate for supporting register with said opposite end face of saidceramic housing, a bolt extending through each said passageway in saidlower plate into threaded engagement with a registered bore in saidupper plate, a head portion on each bolt that is of larger diameter thanthe diameter of said smaller diameter passageway portion and recessedcompletely within the larger diameter passageway portion, each helicalspring coaxial with and surrounding each of said bolts, opposite ends ofeach of said springs being compressed between said facing surfaces ofsaid plates to maintain a generally parallel relationship between saidupper and lower plates and yet allow movement between the plates forequal pressure distribution on the upper plate.

1. In a cold welding apparatus for circumferentially cold welding acircular cover member against one End face of a tubular ceramic housinghaving opposite end faces in which said one end face of said ceramichousing has an integral annular cold welding die formed thereon, ringmeans for registration with said integral annular cold welding die onsaid ceramic housing and for selectively applying cold welding pressureto said die throughout its circumference, a pressure equalizing assemblywhich comprises a pair of spaced plates having generally aligned facinghemispherical recesses, a bearing ball nested within said recesses, theradius of curvature of said bearing ball being generally equal to thatof the recesses, ring means on one of said plates for supporting theopposite end face of the housing, and means for resiliently retainingsaid plates together yet allow said one plate to pivot on said bearingball in order to equally distribute a compressive force on the end facesof the tubular ceramic housing, while the other plate remains stationaryon a support.
 2. In a cold welding apparatus for circumferentially coldwelding a circular cover member against one end face of a tubularceramic housing having opposite end faces in which said one end face hasan integral annular cold welding die formed thereon, ring means forregistration with said integral annular cold welding die on said ceramichousing and for selectively applying cold welding pressure to said diethroughout its circumference, a pressure equalizing assembly whichcomprises spaced upper and lower generally circular plates havingmutually facing and opposite surfaces, said facing surfaces of saidplates having generally aligned hemispherical recesses, said upper platehaving threaded bores extending perpendicularly into said facing surfaceand only partially through said plate, the lower plate havingpassageways therethrough perpendicular to its facing surface and inregister with said threaded bores, each of said passageways having asmaller diameter portion intersecting the facing surface of said lowerplate and a larger diameter portion intersecting the opposite surface ofsaid lower plate, a bearing ball nested within said recesses on saidfacing surfaces of said plates, the radius of curvature of said bearingball being generally equal to that of the recesses, ring means on saidopposite face of said upper plate for supporting register with saidopposite end face of said ceramic housing, a bolt extending through eachsaid passageway in said lower plate into threaded engagement with aregistered bore in said upper plate, a head portion on each bolt that isof larger diameter than the diameter of said smaller diameter passagewayportion and recessed completely within the larger diameter passagewayportion, each helical spring coaxial with and surrounding each of saidbolts, opposite ends of each of said springs being compressed betweensaid facing surfaces of said plates to maintain a generally parallelrelationship between said upper and lower plates and yet allow movementbetween the plates for equal pressure distribution on the upper plate.